The two most widely-used polymer films for food and beverage packaging are polyethylene terephthalate and biaxially-oriented polypropylene. One of the major factors affecting the shelf life of food packaged in containers including films made of the above two materials is their permeability to oxygen and water vapour.
At present, the barrier properties of these materials are improved by vacuum coating them with aluminium to a thickness of a few tens of nanometers and an optical density of about 2.5. Metallization of one side of the film only reduces the oxygen permeation by a factor of about 100 and water vapour permeation by a factor of about 500. Metallization of both sides of the film improves these figures by a further factor of 5.
However, the use of aluminium as a coating material has several disadvantages; one is the possible link between the presence of aluminium particulates in food and Alzheimer's disease; another is that the contents of a package are invisible to a would-be purchaser, a third is that the presence of the aluminium coating means that food which requires cooking cannot be cooked in the package by microwave radiation, and a fourth is that such coated materials cannot be recycled.
Other methods which have been suggested for reducing the permeability of polymer films include laminating together two different polymer films, for example, polyvinylidene chloride and polyethylene terephthalate; depositing a film of silica on the polymer film by the thermal evaporation of silica; or the deposition of other oxides of silicon by various vapour deposition techniques. The purpose of the use of silicon oxides other than silica is to attempt to provide barrier layers which do not suffer from the brittleness of silica.
It is known that the rate of diffusion of fluorinated hydrocarbons through glassy polystyrene can be reduced by bombarding the polystyrene with H.sup.+ ions at an energy of 30 KeV and ion doses between 10.sup.13 and 10.sup.15 ions/cm.sup.2 (see Study of Diffusion of a Fluorinated Hydrocarbon in Ion Beam Irradiated Polystyrene by Nuclear Magnetic Resonance Reaction Analysis. Uwezawa et al Appl. Phys. Lett., Vol. 61(13), Sep. 28, 1992 pp 1603-4). The work was carried out in connection with the development of polymer encapsulants for microelectronic devices.
Another problem associated with polymers as packaging materials is that it is difficult to print upon them. The problem is exacerbated by the transition, for environmental reasons to water-based rather than hydrocarbon solvent-based inks.